Motor control system



Patented Nov. 8, 1932 UNITED STATES "PATENT orrics HARRY "WINNE, 0FSGHENECTADY, NEW YORK, ASSIGN'OR TO GENERAL ELECTRIC COMPANY, ACORIPGRATION 0F NEW YORK MQTOR CONTROL SYSTEM Application filed February'24, 1930. Serial No. 430,465.

My invention relates to systems of motor control, more particularly to afield protective system for electric motors, and has for its object toprovide adequate over-speed protection which will not function underabnormal operation unless there is present a under certain conditions,particularly in the case of a motor having both shunt and series fields,the value of the shunt field current is not a true indication of thefield flux condition in the motor and therefore that a protective deviceresponsive to the shunt field current alone may operate when a low valueof field flux, against which the device is intended to give protection,does not exist. For example, with a cumulative compound motor a suddenincrease in'armature current from any cause, such as might be due to asudden increase in load or to a reversal of the motor armature, producesa corresponding increase in the series field current which in turnincreases the field flux in the motor. Thissudden increase in field fluxproduces a counter-electromotive force in the shunt field winding whichopposes the existing electromotive force in the shunt field winding. Asa result of this induced counter-electromm tive force in the shunt fieldwindings, the current in the shunt field winding may be temporarilyreduced to a very considerable extent even though the total field fluxhas actually increased due to the efi'ect of the series winding. In casethe shunt field current is reduced by this action to the minimum valueto which a protective relay responsive only to this current is set, therelay would thus operate to disconnectthe motor even though the motorfield flux were still adequate for satisfactory operation.

In accordance with my invention I provide means for controlling themotor in response to the field flux conditions actually existing in themotor. In one form of my invention I reproduce the flux conditions inthe motor in an auxiliary control device such as a relay having aplurality of coils, one of which is in series with the shunt fieldwinding and the other in series with the series field winding, wherebythe relay is operated to break the circuit of the motor when the totalflux in the relay has dropped to a predetermined value. It will beunderstood that my invention is not limited to the reproduction of theflux conditions by a relay alone but obviously includes the arrangementof a relay withrespect to the motor so that the relay is responsive tothe motor flux.

My invention will be better understood and further objects will becomeapparent from a study of the following description taken in conjunctionwith the accompanying drawing, the single figure of which is adiagrammatic representation of a system of motor control embodying myinvention.

Referring to the drawing, I have shown my invention in one form asembodied in a control system wherein a motor 10 having a cumulativeseries field 11 and a shunt field winding 12 may be connected throughreversing switches 13 and 14 to the supply line 5. The switches 13' and14. are actuated by windings 15 and 16 which are energized from acontroller 17. Interposed in the series field circuit is a coil 18 on arelay 20 and a coil 19 on the relay is similarly interposed in serieswith the shunt field circuit, these'two coils acting to assist eachother.

The flux in the motor, of'course,.is substantially dependent on themagnitude of the current flowing through its field windings, and thecoils 18 and 19 are so arranged and proportioned that the flux conditionexisting in 'the motor is approximately reproduced in the relay by thecoils.

- The circuits may be traced by first considering the controller 17 in aposition such that the movable segments 21 engage with contacts 22 and23, the line switch 5 being closed. It is to be noted that the shuntfield winding 12 and the coil 19 are permanently connected across theline so that the relay 20 is normally maintained closed by the coil 19.The circuit may now be traced through the conductor 24, the relaycontacts and 26 which are bridged by means of member 27, through wire 28to the controller contact 22, through the segments 21 to contact 23',the conductor 28A, the coil 16 and through wire 29 to the other side ofthe line. The switch 14 is then closed by the coil 16 and completes thecircuit to the armature of the motor, which circuit may be traced fromthe positive side of the line through conductor 30, the relay coil 18,

series field 11, and conductor 31 to one side of switch 14:, thencethrough conductor 32 to the armature of the motor and conductor 33, theother side of switch 14, and conductor 34 to the other side of the line.

When the operator wishes to reverse the motor, the controller 17 ismoved so that segments 35 engage contacts 22 and 36 completing a circuitthrough the coil 15 which closes the switch 13 to complete a circuit .tothe motor armature with the polarity of the current reversed.

With this arrangement it will be observed that upon a sudden surge ofarmature current such as might arise from a sudden application of loador from a reversal of the motor before its. armature has come to rest,the motor field flux conditions are closely reproduced in the relay 20and consequently the relay will be held in the closed position as longas the motor has an adequate field flux for safe running conditionsWhile the current in the relay coil 19 may be greatly reducedtemporarily due to the counter-electromotive force induced in the shuntfield winding 12 as well as in the relay winding 19 itself, the fieldflux in the motor may still be suflicient for safe running due to theeffect of the series field. This condition is substan tially duplicatedin the relay due to the effect of the relay winding 18 and consequentlythe relay does not operate. A motor field flux less than a predeterminedminimum 'for which the relay is arranged causes the relay to drop out,however, and deenergize the motor.

While I have described a particular'embodiment of my invention, andcertain apparatus therefor it will be obvious to those skilled in theart that various modifications may be made without departing from theSpirit of my invention. I therefore desire to cover all suchmodifications as fall within the scope of my invention which is'setforth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The combination with a motor provided with shunt and series fieldwindings, of means operated in accordance with the total flux producedby said windings after the motor has been accelerated to a running speedfor controlling said motor.

2. The combination with a dynamo electric produce the field fluxconditions of said motor in said control device.

3. The combination with an electric motor provided with shunt and seriesfield windings, of a protective relay for controlling the energizationof said motor, and a pair of actuating coils for said relay connectedrespectively in series with said field windings during running operationof said motor so as to substantially reproduce the field fiux conditionsof said motor in said relay, and thereby cause said relay to deenergizesaid motor upon the occurrence of a motor field of a predetermined lowvalue.

4. In a protective device for a motor having shunt and series fieldwindings, means for reversing the direction of current flow to thearmature circuit to reverse the rotation of the motor, and meansresponsive to the flux produced in the motor by said fieldwinding'during running operation of said motor for controlling saidreversing means to deenergize the motor when the total flux decreases toa predetermined low value.

5. The combination with an electric motor provided with a shunt fieldwinding and a series field winding, switching means for controlling theenergization of said motor for each direction of rotation, a controldevice for said switches, and operating coils for said control deviceconnected respectively in circuit with said field windings so as tosimulate the field conditions in said motor during running operation ofsaid motor and release said control device upon the occurrence ofpredetermined low field strengths.

6; The combination with an electric motor provided with a shunt fieldwinding and a series field winding, electromagnetically ac tuatedreversing switches for controlling the energization of said motor, acontrol circuit for said switches, a relay switch in said controlcircuit, and actuating coils for said relay permanently connectedrespectively in sesaid control circuit upon the occurrenceof apredetermined low field flux strength.

7. The combination with a motor provided with shunt and series fieldwindings, of circuit control means for said motor and means operated 'inaccordance with the total flux produced by said field windings duringrunning operation of said motor for controlling said circuit controlmeans to deenergize said meagre 3 motor upon the occurrence of apredetermined reduced motor field.

8.. The combination with an electric moator provided with shunt andseries fieid Wind- 5 ings, of a control device for said motor providedWith windings permanently connecised in series With said field windingsrespec civeiy, said windings being arranged to jointly release saidrelay to deenergize said motor 10 upon the occurrence of a predeterminedweakened motor field.

In Witness whereof, I have hereunto set my hand this 22nd day ofFebruary, 1930.

- HARRY A. WINE

